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gatt (Mechanical) (OP)
31 Aug 05 14:25
We are going to replace 2 reciprocating compressors on a chiller unit. These compressors share a common circuit. Outdoor air cooled condensers, chiller barrels inside below condensers. One contractor wants to install suction accumulators and oil separators. Two other contractors would not bid on either of those items. I'm leaning towards installing accumulators (~8% costlier) and forgetting about separators (~25% costlier). Any suggestions?

Thanks, Gatt
imok2 (Mechanical)
31 Aug 05 15:52
In my humble opinion, you don't need these items unless you have been having problems. I look at it as insurance but costly. Oil separators “are essential” on low or ultra-low temperature refrigeration systems.  Oil separators can also act as discharge mufflers to quiet compressor pulsation and vibration noises. In addition, unusual system operating conditions can occur to compressors and rapid removal of oil from the compressor’s crankcase happens. System flooding is an example. Do not ever undersize an oil separator because it will loose it’s ability to return oil and will cause high pressure drop causing system inefficiencies. Helical oil separators offer 99 percent to 100 percent efficiency in oil separation with low pressure drop. Most oil separators must be insulated to keep them hot during the on and off cycles.
Liquid Accumulators
Liquid flood-back is liquid refrigerant returning to the compressor’s crankcase during the running cycle. Refrigerant migration is refrigerant liquid or vapor returning to the compressor during the off cycle. Migration usually happens because of a pressure difference. The compressor can be located in a cold ambient causing a lower pressure, or the oil can be attracting the refrigerant because of its lower vapor pressure. In either case, the refrigerant will migrate to the compressor because of its lower pressure. The causes of compressor floodback include:
 
·  Wrong TXV setting (low or no compressor superheat);
·  Overcharge;
·  Low load on evaporator;
·  End of cycle (lowest load);
·  Dirty or blocked evaporator coil;
·  Expansion bulb loose on evaporator outlet;
·  Oversized expansion valve;

Suction line accumulators are designed as compressor protection devices when flooding and migration do occur. So depending on your particular application you need to look at the above and determine whether these are possible conditions you will have.

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